I'm unable to reproduce this with the revision + shell flags in comment 0, x86 debug build on OS X.
NI from decoder, this one shouldn't be too hard to figure out once I have access to a machine where this reproduces.

This also needs --ion-parallel-compile=off to reproduce reliably. It does appear to be backtracking only.
Here's a simplified testcase that also fails with --ion-eager --ion-regalloc=backtracking --ion-parallel-compile=off on x86 linux:
(function () {
var s = "{}";
for (var i = 0; i < 19; i++)
s += s;
eval(s);
})();
The OsiPoint that triggers this error is the one immediately after the Concat. The mismatch is with %edx, which is Concat's output register.